#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
using namespace std;
#include<vector>

class Solution {
public:
    int tribonacci(int n) {
        vector<int> dp(38);
        dp[0] = 0, dp[1] = dp[2] = 1;
        if (n < 3) return dp[n];
        for (int i = 3; i <= n; i++)
        {
            dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3];
        }
        return dp[n];
    }
};

class Solution {
public:
    int waysToStep(int n) {
        vector<int> dp(n + 1);
        dp[0] = 0, dp[1] = 1, dp[2] = 2, dp[3] = 4;
        if (n == 1 || n == 2) return 0;
        if (n == 3) return 4;

        for (int i = 4; i <= n; i++)
        {
            dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3];
        }
        return dp[n];
    }
};

#include<algorithm>
class Solution {
public:
    int minCostClimbingStairs(vector<int>& cost) {
        int n = cost.size();
        vector<int> dp(n + 1);
        dp[0] = dp[1] = 0;
        for (int i = 2; i <= n; i++)
        {
            dp[i] = min(dp[i - 1] + cost[i - 1], dp[i - 2] + cost[i - 2]);
        }
        return dp[n];
    }
};

class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        vector<int> dp(n);
        if (s[0] != '0')
        {
            dp[0] = 1;
        }
        if (n == 1) return dp[0];
        if (s[0] != '0' && s[1] != '0')
        {
            dp[1] += 1;
        }
        int t = (s[0] - '0') * 10 + s[1] - '0';
        if (t >= 10 && t <= 26)
        {
            dp[1] += 1;
        }
        for (int i = 2; i < n; i++)
        {
            if (s[i] != '0')
            {
                dp[i] += dp[i - 1];
            }
            int t = (s[i-1] - '0') * 10 + s[i] - '0';
            if (t >= 10 && t <= 26)
            {
                dp[i] += dp[i-2];
            }
        }
        return dp[n - 1];
    }
};